1. Field of the Invention
This invention relates to a polymerization method for producing polymers such as polyesters and a apparatus used therefor.
2. Description of the Related Art
In production of polymers, it is a common practice to measure the viscosity of the reaction mixture so that the progress of the reaction is estimated from this viscosity to control the reaction. For example, part of the reaction mixture is taken from a reaction vessel and sent to a viscometer consisting of a constant delivery pump and a capillary, where the measured differential pressure is converted to a viscosity, and the process is controlled based on the thus determined viscosity.
In a rotary polymerization apparatus, the process could be monitored by measuring the viscosity of a sample of the reaction mixture as in a continuous production apparatus, but such is not practical taking into account the time lag, change-over of the species to be produced, cleaning of the reaction vessel, and the like. Instead, the process control is usually effected by detecting the load imposed on the stirrer in the reaction vessel and calculating the viscosity of the reaction mixture from the load so as to obtain product consistency based on the calculated viscosity.
It is desirable that the load applied to a stirring machine is obtained by measuring the torque on the central shaft of the stirring machine with a torque meter directly attached to the shaft. However, because the size of the torque meter must be increased in proportion to the size of the reaction vessel, i.e., the size of the stirring machine, the positions of fixing equipment onto the polymerization vessel are restricted.
Therefore, it is the load imposed to the rotating motor shaft driving the stirring machine that has been measured to calculate the viscosity of the reaction mixture. Since the load on the motor shaft is smaller than that on the stirring machine itself, the torque meter can be made smaller (see, e.g., Examined Japanese Patent Publication No. Sho. 53-24233).
There being no large torque meter right above the reaction vessel, various pipes, a monitoring window, a hand hole for cleaning, a header for pressure application, and the like can be attached to the reaction vessel at positions easy to handle.
However, the torque measured on the rotating shaft of the motor consists of not only the resistance of the reaction mixture but also, as hereinafter described, the resistance of lubricating oil used in a driving mechanism (a transmission box and a speed reduction gear) which greatly varies depending on the surrounding temperature or the running time, thereby impairing the measurement accuracy.
The viscosity of a lubricating oil widely fluctuates with temperature as shown in FIG. 4. For example, when the outdoor temperature falls to 5.degree. C. or below in winter, the viscosity rises to 1000 cSt or higher. The temperature of the lubricating oil can exceed 80.degree. C. in summer when the transmission box and the speed reduction gear are operated for a long period of time or when a highly viscous reaction mixture is stirred, in which the viscosity of the lubricating oil can drop to 20 cSt or lower.
FIG. 5 depicts the relationship of power loss vs. running time in a transmission box and a speed reduction gear using various lubricating oils a1, a2, b1, b2, c1, and c2, with no load applied with no reaction mixture in a reaction vessel. The power of the motor is 15 kW. It is seen that the power loss is very large in the beginning of stirring irrespective of the kind of the lubricating oil and that more than a half the motor power is consumed by the transmission box and the speed reduction gear. The power loss is reduced with running time, ultimately reaching a stationary state. This is because the temperature of the lubricating oil increases with running time, which reduces the viscosity.
Where several batches are polymerized in a rotary polymerization apparatus using lubricated transmission box and speed reduction gear, a great power loss occurs in the first batch due to the low lubricating oil temperature in the speed reduction gear and transmission box. As a result, cases are sometimes met with, in which the actual viscosity (intrinsic viscosity; hereinafter abbreviated as IV) of the reaction mixture withdrawn from the reaction apparatus is unexpectedly lower than the one calculated from the measured torque on which the operator has made a judgement in deciding when to stop the reaction.
In order to avoid such a situation, it is necessary to reduce the viscosity of the lubricating oil by increasing the lubricating oil temperature by operating the transmission box and speed reduction gear beforehand. However, this method involves waste of time and power.
Because a lubricating oil changes its viscosity between winter and summer, it has also been proposed to use a low-viscosity lubricating oil in winter to reduce a power loss and a high-viscosity lubricating oil in summer to prevent breaking of an oil film.
However, when the reaction is controlled while calculating the viscosity of the reaction mixture from the motor torque measured even with such a measure for power loss reduction being taken, the resulting resin often shows variation of intrinsic viscosity (IV) from batch to batch. Thus, it has been required that the resin from different batches should be blended to level the IV variation, which results in reduction of productivity.